Conventional antenna arrays, as used in cellular infrastructure macro cells, comprising multiple antenna elements and used with existing Node-B equipment in most third generation (3G) installations, utilise a fixed 65° beam pattern. Outside of the main lobe of the antenna beam the signals are spatially filtered and significantly attenuated. Conventional network planning and passive antenna array solutions process all incoming signals with a common fixed beam pattern. Such receive processing, based on signals received within the geographic area identified by the antenna beam main lobe, referred to as the RF footprint, tends to dictate a corresponding common beam pattern for transmitter operation. Thus, an identical radio frequency (RF) footprint is used for both receive (Rx) and transmit (Tx) operation.
HSPA+, also known as Evolved High-Speed Packet Access is a wireless broadband standard defined in 3GPP release 7 and is an evolution of the third generation (3G) cellular communication standard based on frequency division duplex (FDD) wideband code division multiple access (WCDMA) technology. HSPA+ provides HSPA data rates up to 56 Mbit/s on the downlink and 22 Mbit/s on the uplink employing multiple input and multiple output (MIMO) and high order modulation (64QAM) schemes. Recent trials in HSPA+ networks have uncovered a problem with capacity and coverage issues with single antenna UE (User Equipment) devices. The intention of HSPA+ is that it should be backward compatible to all network UEs including those supporting just HSDPA and Release 99 versions of the 3G standard. HSPA+ introduces and utilises transmit diversity on the Node-B network element.
Network Operators prefer to use polarisation diversity for MIMO transmission on HSPA+, such that MIMO signals share the same frequency but different data is modulated on to respective carriers as transmitted over different polarisations. Polarisation diversity is preferred over spatial diversity as the antenna can be used at the top of the antenna mast, as for previous versions of the 3G standard. Furthermore, many sites are crowded and room for extra antennae is not available. Field trial results have also shown that the equivalent or better MIMO link gains can be found through use of polarisation diversity only.
Network operators and 3GPP standards now intend to use a primary common pilot channel (P-CPICH) on one of the virtual antenna mapped transmissions and a secondary CPICH (S-CPICH) on the other. CPICH is used by UE devices in the rake receiver for both the channel equalisation and rake receiver channel estimator. In the absence of a CPICH, for example if it is not transmitted from the node-B, alternate equalisation and rake receiver channel estimator techniques may be employed. Usually an algorithm such as a minimum mean square error (MMSE) algorithm is used to estimate the weights and delays of the Rake receiver in WCDMA based receptions without the CPICH being present.
Many current UEs, will not support new upgrades to the 3G standard and are therefore unable to utilise HSPA+. In particular, recent trials of HSPA+ networks have uncovered a problem due to a use of linear polarisation (LP) transmission diversity and its effects on 3G UE devices that do not have the capability of diversity reception. A UE device supporting only older versions of the standard may only have one receive antenna and, thus, will not be able to exploit the transmission diversity of the upgraded network. Such a UE device will obtain its call traffic routed through one of the node B transmit diversity paths only. A problem arises as the LP antenna UE device is rotated or moved to a location where the second orthogonal transmission from the MIMO enabled Node B becomes much stronger than the desired first orthogonal transmission. This second orthogonal transmission signal then exhibits itself as an uncorrelated noise-like interferer on the UE receiver receiving the first orthogonal transmission. Furthermore, the second orthogonal transmission signal remains as an uncorrelated interferer as such a UE device is not able to process both MIMO transmissions at the same time. The received carrier to interference plus noise ratio (CINR) may degrade the receiver performance by 10's of dBs, thereby causing communication links to be dropped and consequently reducing cell coverage area.
If the MIMO transmission is left-hand circularly polarised (LHCP) and right-hand circularly polarised (RHCP), as opposed to LP +45° and LP −45° polarisation, then the impact on legacy 3G UE devices is reduced. This is because the signal to interference due to the second MIMO carrier is substantially limited to 3 dB, i.e. the signal of both LHCP and RHCP are the same power for all orientations of the UE device antenna. Thus, HSPA+ enabled UE devices do not have their reception adversely affected by use of a CP signal.
Since orthogonal LHCP and RHCP antennas for MIMO (Multiple Input Multiple Output) transmission in network trials has proven to be successful in reducing this problem with single antenna UE devices, this implies that an antenna for the node B must be capable of concurrent transmission in LHCP and RHCP for virtual antenna mapped signals.
The HSPA+ protocol is currently being designed to support a technique known as Virtual Antenna Mapping (VAM). This technique was originally intended to help with equalising the power payload for both transmitters delivering the MIMO signals to the sector. VAM in the version 7, Release 2010 of the HSPA+ standard limits the precoding values of the virtual antenna mapping. These precoding values are equivalent to performing a 90° phase shift to one path in the digital domain, with a similar effect to that of placing a 90° 3 dB hybrid coupler in the RF domain paths.
Referring now to FIG. 1, examples of known electromagnetic waveforms are illustrated. A first diagram 100 illustrates a linear polarised field from an antenna and a second diagram 150 illustrates a circular polarised field. The polarization of an antenna is the orientation of the electric fields (E-plane) 110 of the radio wave with respect to the Earth's surface and is largely determined by the physical structure of the antenna and by its orientation. The magnetic field (H-plane) 120 is always perpendicular to the E-plane 110. The E-plane 110 and H-plane 120 are respectively illustrated as propagating in the directions 105, 115. In contrast, circular polarised (CP) antennas as illustrated in the second diagram 150 have a rotating E-plane 160 in a propagation direction 155, in contrast to the linear polarised (LP) antennas having a fixed E-plane.
Circular polarisation is the polarisation of electromagnetic radiation, such that the tip of the electric field vector describes a circle in any fixed plane intersecting, and normal to, the direction of propagation. However, in practical systems there will be minor deviations from this perfect angular electric field vector that describes a circle. For the purposes of the description hereinafter described an E-Field vector that is substantially close to that of a circle is considered to be a circularly polarised field.
Elliptical polarisation is the polarisation of electromagnetic radiation, such that the tip of the electric field vector describes an ellipse in any fixed plane intersecting, and normal to, the direction of propagation. Elliptical polarised fields can be configured as circularly polarised fields, and can be rotated polarised fields in a clockwise or counter clockwise direction as the field propagates; e.g. forming right hand elliptical polarisation and left hand elliptical polarisation respectively. An elliptically radiated field will have substantially changed magnitude for 90° change in angular vector.
Cross-polarisation (XPOL) antennas are also often used, particularly in cellular infrastructure deployments. XPOL antenna technology utilises pairs of two LP antenna elements that are orientated substantially 90° with respect to each other, often referred to as being ‘orthogonal’ to each other, usually at +45° and −45° polarisation. These pairs are often elements in an array, and thus can be arranged such that a desired propagation beam shape is developed. To date, deployed cellular infrastructure transmit polarisation orientation predominantly only uses one of the polarisation types whereas receive functionality is performed in both polarisations, with separate and independent processing of the two XPOL receive paths being employed. These XPOL antennas can be of patch construction (PCB) or of Dipole (Wire) construction. Currently, some Network Operators are supporting HSPA+ using two polarisations for the transmission of MIMO signals.
A known problem in using LP transmissions is that the polarisation of the transmitted signal antenna and the receiving signal antenna (if also an LP type) needs to have the angle of polarisation exactly the same for reception of the strongest signal. For example a signal transmitted on a vertically polarised (VP) antenna and received on an antenna with horizontal polarised (HP) may have 10's of dB difference in received power compared to a matched VP antenna. Mobile handset antennas are generally LP, though increasingly through means of diversity reception paths a second polarisation diversity LP antenna is utilised, orthogonally polarised to the first.
However, all existing antenna infrastructure is of a linear cross-polarisation type. There is a need to convert signals being fed to a cross polarisation (XPOL) antenna and modify them such that they can be broadcast in CP modes using existing antenna infrastructure. A CP signal can be generated in a XPOL LP antenna arrangement by splitting the power in two and adding a 90° phase rotation to one of the paths with respect to the other. In order for a signal to be transmitted as a CP signal the 90° phase relationship has to be maintained from baseband right through to the antenna elements. CP polarisation is sensitive to the phase difference between the signals at the radiating elements. This correction can be compensated for in the RF domain or in the baseband processing domain. In the RF domain the 90° phase rotation to one of the paths is often achieved with a 3 dB hybrid coupler. Internal feeds to XPOL elements of respective +45° and −45° polarisation are not specified or controlled to be matched electrical lengths on existing antennae. Furthermore, cable feeds from the base station or remote radio head to the antenna are typically cut to measure and installed in the field. Consequently, a phase relationship of signals applied to the orthogonal antenna elements is unknown.
Where XPOL LP antennas are used to radiate CP signals the phase to the antenna elements needs to be tightly controlled. As a polarised signal may deviate from its ideal 90° difference, then the polarisation diversity benefits deteriorate quickly to an elliptical type polarisation, thus greatly affecting the performance of communications in the network. The resultant phase imbalance may be accumulated at multiple sources, which include for example, cable feeds, transmitter chains, phase locked loops (PLLs), duplexers, etc. Thus, the phase at the output of the base station/Node-B is not controlled. Normally there will be a non-deterministic phase offset at output ports due to process variation, temperature profile, carrier frequency and initial conditions, etc. at a power-on of internal components of the base station/Node-B. Where Virtual Antenna Mapping is used, these phase offset sources will cause the phase of the signals presented to the MIMO calibration unit to be non-deterministic and to change due to environmental conditions.
Simple measurement and phase adjustment techniques cannot be used to correct for the above problems, as the termination of the antenna feeds affecting the signal paths is actually made inside the antenna array, i.e. at the radiating elements, and these can not be accessed in an electrical type test. Furthermore, the phase shift may be frequency dependant, especially if there is significant mismatch in cable lengths. In laboratory tests, it has also been found that a difference in torque applied to the cable connectors has a significant impact on the phase response, which can be as much as seven degrees per connector. Thus, any measurements performed prior to installation are insufficient to accurately set phase shift circuitry in the network element prior to the antenna/antenna array. Also, for the above reasons a use of a single phase setting is incapable of guaranteeing an accurate phase of polarisation signals from the antenna/antenna array. Furthermore, offsets due to environmental changes cannot be compensated for in an electrical test before installation.
To date no known solution has been developed to determine, or correct for, a phase mismatch of signals coming from an antenna on respective MIMO feeds right through to translation of signals of orthogonal components on the transmission path. Furthermore, there is no known proposal to adjust VAM generated signals, for example when applied to HSPA+ networks.
U.S. Pat. No. 4,737,793 discloses a microstrip-based XPOL antenna element with two 3 dB hybrid couplers and four radio frequency phase shifters. There is no mention of any adjustment of the phase shifter for the purpose of offsetting mismatch in cable feeds. U.S. Pat. No. 4,737,793 provides no teaching of either a calibration method or a feedback technique, for example using feedback couplers for sensing and updating the phase shifter settings. Furthermore, the use of excessive processing on the signals at the antenna is undesirable, as the losses induced would be excessive and cause noise figure degradation of the receiver performance and an unacceptable loss on the PA output for transmission. In addition, the teaching of U.S. Pat. No. 4,737,793 does not propose any exploitation of VAM aspects of the air interface protocol layer to determine phase mismatch.
U.S. Pat. No. 6,262,690 proposes a use of a hybrid coupler and a phase shifter at the input to an amplifier pair to adjust a phase of a signal fed to a single antenna element via an orthomode transducer, which is a device that separates signals received from an antenna into their respective received polarisation types. The phase shifters are employed to correct for phase offsets induced by the amplifiers. In addition, U.S. Pat. No. 6,262,690 does not propose any exploitation of VAM aspects of the air interface protocol layer to determine phase mismatch.
Furthermore, receiver examples using active panel antenna technology, as exemplified by co-pending application GB0921956.9, utilise a receiver to calibrate and compensate for any phase mismatch between respective antenna feeds of different polarisations to an antenna array. In such examples, the compensation mechanism has to refer back to altering the transmission signal in the digital domain, which is not always possible particularly where the antenna element is physically far removed from the baseband signal generation, which is typically the case in most Node B equipment.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of supporting antenna array technology in a wireless communication network would be advantageous.